Polar Regeneration in Excised Roots of Taraxacum officinale Weber: A Light and Electron Microscopic Study

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Abstract
The day 0 secondary phloem of Taraxacum officinale root segments contains wide bands of parenchyma alternating with thin cylinders of conducting tissue composed of discreet conducting strands. At day 1 in the inner distal phloem (and by day 2 proximally) the initially-flattened nuclei of some parenchyma cells become rounded, more densely stainable and a few have migrated from the peripheral cytoplasm to a suspended position in the vacuole. Cell division occurs asynchronously and gradually extends to the midphloem. By day 4 nodules of primary meristematic cells occur proximally and numerous young leaves are visible externally at days 5–6. Distally, callusing of the phloem is more extensive and links with that developing from the secondary xylem. Proximally adventitious buds form and these are both more abundant and quicker growing than the distally located adventitious roots. Although proliferation is initially mainly confined to the companion cells it increasingly involves activation of the parenchyma tissue. These cells undergo a cytological de-differentiation with the daughter cells showing a progressive decrease in cell size and vacuome (with cytoplasmic strands, perhaps indicative of lysosomal activity, often visible in the vacuoles), accompanied by an increase in nucleolar size and cytoplasmic density.

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